题目

It was the ambition of an eleven year old boy in Kansas City to be an engine­driver. Born without arms, he had been sent to special schools 41.          he learned to use his 42.         (foot) as “ hands”. He spent all his spare time 43.         (watch) trains and one day his dream came 44.        (truth). Seeing a deserted engine, the boy climbed in, he had no 45.         (difficult) in starting it up with his feet. Soon he was traveling along 46.         forty miles an hour. Signalmen could not see the young driver, so they set o某气态有机物X含C、H、O 3种元素，现已知下列条件：①X中含碳质量分数；②X中含氢质量分数；③X在标准状况下的体积；④X对氢气的相对密度；⑤X的质量。现欲确定X的分子式，所需要条件最少的是(    ) A.①和②               B.①②④                  C.①②⑤                 D.③④⑤